1. Field of the Invention
The present invention relates to an optical modulator, a method for fabricating that optical modulator, and a photonic semiconductor device. More particularly, the invention relates to an optical modulator used in optical communication, a method for fabricating that optical modulator, and a photonic semiconductor device that combines such optical modulators.
2. Description of the Related Art
In order to promote widespread use of public communication networks with optical fibers, it is important to boost the performance of semiconductor laser devices and enhance their yield for less costly fabrication. In particular, improving semiconductor laser performance necessarily involves modulating laser emissions at higher speed so as to deal with growing quantities of information. Optical communication over long distances is implemented by minimizing wave fluctuations during high-speed laser modulation, whereas conventional setups having an injected current varied in single-mode semiconductor laser for direct modulation tend to suffer a pronounced wavelength chirping caused by fluctuating densities of injected carriers. For that reason, the direct modulation scheme cannot be used in long-distance high-speed modulated data transmissions at 10 Gbps or higher.
For 10-Gbps optical data transmission systems, the direction modulation scheme is typically replaced by an external modulation setup. External modulation involves keeping a semiconductor laser device oscillated at a constant level and having the emitted laser pass through optical modulators capable of turning on and off light transmission with a minimum of wavelength chirping in order to achieve light modulation.
Optical modulators used by the external modulation method are called electro-absorption modulators, abbreviated to EAMs hereunder. EAMs having a single optical absorption layer absorb light through the use of the Franz-Keldysh effect, and EAMs with a multiple quantum-well structure absorb light through absorption spectrum variations based on the Stark effect. Laser absorbency of an optical modulator varies depending on a backward bias voltage applied to the modulator in question. For that reason, if a modulation signal voltage is applied to a high-frequency electrical circuit connected to an optical modulator, a laser beam that is modulated in intensity reflecting the signal voltage is emitted from an emission face of the optical modulator.
In the field of high-speed communications at 20 Gbps or higher of the next generation, ultra-high-speed semiconductor optical modulators are drawing attention because they have a low-chirping characteristic, are small in size, and operate on low voltages. Implementing such ultra-high-speed semiconductor optical modulators faces an important challenge: how to minimize the capacity of optical modulator elements.
FIG. 25 is a perspective view of a conventional optical modulator, and FIG. 26 is a cross-sectional view taken on line XXVIxe2x80x94XXVI across the optical modulator in FIG. 25. In FIGS. 25 and 26, reference numeral 200 stands for an optical modulator; 202 for an n-type InP substrate (n-type conductivity is denoted by a symbol xe2x80x9cn-xe2x80x9d and p-type conductivity by xe2x80x9cp-xe2x80x9d hereunder); 204 for an n-InP clad layer; 206 for an optical absorption layer; 208 for a p-InP clad layer; 210 for a p-InGaAs contact layer; 212 for a surface protective film such as an SiO2 film; 214 for a polyimide layer; 216 for a p-type ohmic electrode; 216a for a bonding pad; and 218 for an n-type ohmic electrode.
A method for fabricating conventional optical modulators is outlined below. FIGS. 27, 28 and 29 are cross-sectional views of an optical modulator fabricated in sequence. On the n-InP substrate 202, the n-InP clad layer 204, the optical absorption layer 206, p-InP clad layer 208, and p-InGaAs contact layer 210 are first formed by epitaxial growth. An insulating film such as an SiO2 film is then formed over the surface on which is provided a stripe-shaped mask pattern 220 measuring 2 to 3 microns (xcexcm) wide (see FIG. 27).
With the mask pattern 220 used as a mask, dry etching is carried out to a depth beyond the optical absorption layer 206, illustratively 2 to 3 microns deep, so as to form a ridge 222 (see FIG. 28). The surface protective film 212 such as an SiO2 film is formed next. Polyimide 214 is applied over the film to flatten the surface. An opening 224 is formed on top of the ridge 222 for ohmic contact (see FIG. 29).
The p-type ohmic electrode 216 and n-type ohmic electrode 218 are then formed, which completes the optical modulator shown in FIGS. 25 and 26. The element capacity of the optical modulator 200 thus fabricated is given as a sum of the capacity of the optical absorption layer 206 and the capacity of the bonding pad 216a. Because the capacity of the optical absorption layer 206 is determined by the performance of modulator elements complying with the dynamic range and extinction characteristic of the optical modulator 200, the element capacity can only be reduced to a certain extent.
When the area for accommodating bonding wires is taken into consideration, the bonding pad 216a may be reduced in area to about 50 xcexcmxc3x9750 xcexcm at most; further reduction of the pad area is difficult to achieve. For that reason, the bonding pad 216a is formed on the surface of the insulating polyimide 214 in order to minimize the capacity of the bonding pad 216a. 
However, optical modulators designed to execute modulation at speeds as high as 40 Gbps or more are required to have an element capacitance of 0.1 pf or less. In the conventional optical modulator structure, the element capacitance is reduced using a thicker polymide layer 214. This has posed a problem: the polymide layer 214 is difficult to form.
Japanese Patent Laid-open No. Hei 3-263388 discloses an optical modulator related to this invention. The disclosed optical modulator has a mesa stripe of a semiconductor multi-layer structure containing active layers, the mesa stripe being flanked by InP high-resistance layers. This optical modulator has its element capacity reduced by use of an air-bridge structure that connects the top of the mesa stripe with a bonding pad on a high-resistance semiconductor substrate. The disclosed optical modulator has a ridge structure different from that of the optical modulator of the invention, to be described below.
The present invention has been made in view of the above circumstances, and a first object of the invention is to provide an optical modulator offering excellent high-frequency performance while being lowered in element capacity.
According to one aspect of the invention, there is provided an optical modulator comprising: a semi-insulating semiconductor substrate with a principal plane partially including an exposed surface; an optical waveguide ridge which is disposed on said semiconductor substrate and which includes a first clad layer of a first conductivity type, an optical-absorption layer, and a second clad layer of a second conductivity type, said optical waveguide ridge further having a side with a flat portion extending uniformly from a top of the ridge to said semiconductor substrate, the flat portion being in contact with the exposed surface of said semiconductor substrate; a dielectric film which covers said optical waveguide ridge and said semiconductor substrate and which has a first opening made at the top of said optical waveguide ridge and a second opening made in a region of said semiconductor substrate other than the exposed surface; a first electrode disposed on said dielectric film and mounted through said first opening on the top of said optical waveguide ridge, said first electrode further extending on the flat portion of said optical waveguide ridge while in close contact with a surface of said dielectric film, said first electrode further having one end thereof established on said semiconductor substrate through the exposed surface thereof; and a second electrode disposed on said semiconductor substrate and connected to the first clad layer through the second opening of said dielectric film.
Accordingly, the inventive structure reduces the capacity of the bonding pad of the first electrode, whereby an optical modulator of excellent high-speed performance is constituted.
Anther object of the invention is to provide a photonic semiconductor device comprising optical modulators of advanced high-frequency performance.
According to another aspect of the invention, there is provided a photonic semiconductor device comprising: an optical modulator having; a semi-insulating semiconductor substrate with a principal plane partially including an exposed surface, an optical waveguide ridge which is disposed on the semiconductor substrate and which includes a first clad layer of a first conductivity type, an optical-absorption layer, and a second clad layer of a second conductivity type, the optical waveguide ridge further having a side with a flat portion extending uniformly from a top of the ridge to said semiconductor substrate, the flat portion being in contact with the exposed surface of the semiconductor substrate, a dielectric film which covers the optical waveguide ridge and the semiconductor substrate and which has a first opening made at the top of the optical waveguide ridge and a second opening made in a region of the semiconductor substrate other than the exposed surface, a first electrode disposed on the dielectric film and mounted through the first opening on the top of the optical waveguide ridge, the first electrode further extending on the flat portion of the optical waveguide ridge while in close contact with a surface of the dielectric film, the first electrode further having one end thereof established on the semiconductor substrate through the exposed surface thereof, and a second electrode disposed on said semiconductor substrate and connected to the first clad layer through the second opening of the dielectric; and a semiconductor laser device aligned in optical axis with the optical absorption layer of the optical modulator.
Accordingly, the invention thus provides a photonic semiconductor device of outstanding high-frequency characteristics.
A further object of the present invention is to provide a method of simplified steps for fabricating an optical modulator offering enhanced high-frequency performance with a small element capacity.
According to a further aspect of the invention, there is provided an optical modulator fabricating method comprising the steps of: forming firstly a first clad layer of a first conductivity type, an optical absorption layer, and a second clad layer of a second conductivity type on a semi-insulating semiconductor substrate; forming secondly by photolithograpy and etching an exposed surface of the semiconductor substrate as well as an optical waveguide ridge which has a side with a flat portion stretching uniformly from a top of the ridge to the semiconductor substrate, the flat portion being brought into contact with the exposed surface of the semiconductor substrate; forming thirdly a dielectric film over the semiconductor substrate and a first and a second opening through the film, the first opening being made at the top of the optical waveguide ridge, the second opening being made in a region of the semiconductor substrate excluding the exposed surface thereof; forming fourthly a first electrode through the first opening on the top of the optical waveguide ridge in such a manner that the first electrode extends along the flat portion of the optical waveguide ridge while in close contact with a surface of the dielectric film, the first electrode further having one end thereof formed on the semiconductor substrate through the exposed surface thereof; and forming fifthly a second electrode connected to the first clad layer through the second opening of the dielectric film.
Accordingly, the inventive method permits fabricating an optical modulator of a reduced element capacity using simplified steps, whereby an inexpensive optical modulator of excellent high-speed characteristics is provided.
Other objects and advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.
Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings.